The term “motor” is represented as both the term “electric equipment” and the term “electric machine”. That is to say, a motor has both the electrical characteristic and the mechanical characteristic. On the design of an induction motor, as the ribs of a rotor is thinner, leakage is smaller, so that the electrical characteristic becomes superior.
However, when the ribs are thin, a bar breaks away on a high-speed rotation to deteriorate the mechanical characteristic. In contrast, as the ribs are thicker, the mechanical characteristic (i.e. the stability of the rotor) is improved, but leakage increases to deteriorate the electrical characteristic. Therefore, it is necessary to design ribs in which both the electrical characteristic and the mechanical characteristic are taken into consideration.
In general, the rotor of an induction motor includes stacked electric steel sheets, a rotor bar, and an end ring. Such a rotor is configured in such a manner as to first manufacture a rotor core having a plurality of slots formed therethrough by stacking a plurality of punched electric sheets, to form a rotor bar by filling each slot of the rotor core manufactured as above with molten aluminum in a die-casting operation work, and to form an end ring connected between both ends of the rotor bar.
When such a rotor bar is manufactured in a die-casting scheme using aluminum, the efficiency thereof is slightly lower than that in the case of using a copper (Cu) bar. Generally, centrifugal force is proportional to weight. However, on comparison with a copper bar, since aluminum, which is the lightest metal, has a 3.3 times lower density, the centrifugal force is small although the rib is configured to be thin, so that the mechanical characteristic is excellent.